The activity of intracellular lactic acid dehydrogenase (LDH) and succinic acid dehydrogenase (SDH) was studied by cytochemistry in BGC-823 gastric carcinoma cell line treated with inosine. Cytoplasmic granules of LDH were found decreased or completely disappeared, while little change of SDH was observed. The tumor cells were decreased in number and became atrophic, especially when the cells were treated with high concentrations of inosine. The results suggest that inosine can play a role in suppressing the activity of intracellular LDH, thus blocking the major source of energy supply of cancer cells.